The economical operation of any power plant or diesel engine is dependent upon efficient energy conversion. Peak efficiency is achieved when fuel combustion efficiency is greatest resulting in maximum utilization of the energy of the fuel. At the same time, complete combustion prevents carboning of the heat exchanger walls in steam boilers or cylinder and valve chambers of diesel engines, thus reducing maintenance coats. For a given fuel burner design, the combustion efficiency is affected by the viscosity of the fuel and, therefore, it is important that the fuel viscosity be controlled accurately.
In addition, efficient operation of the pump which supplies the fuel to the power plant or diesel engine is dependent upon the viscosity of the fuel. If the viscosity is too high, the delivery pressure of the pump may become much too great and an excessive amount of energy will be required to operate the pump.
One way to maintain relatively constant viscosity is to use a 2 diesel fuel which has characteristics at ambient temperature conditions which are ideal for efficient use. However, such fuel is very expensive.
A very common practice today is to use a cheaper grade of fuel and to control its viscosity by heating the fuel in accordance with in-line measurements of different characteristics of the fuel. One such practice involves measuring the fuel temperature as it leaves a steam heated heat exchanger and varying the amount of steam being passed through the heat exchanger to vary the fuel temperature and thereby control the fuel viscosity. Such an approach, however, is inadequate in that for a given grade of fuel, a particular temperature may result in a range of viscosities because of variations in the production of the fuel.
Thus, a preferred approach is to measure the fuel viscosity directly with a viscometer which is inserted in the fuel line and to automatically control the heater according to the viscosity measurements. U.S. Pat. No. 2,771,770 shows a viscometer having a pump which forces a portion of the fuel passing through a fuel line through a capillary tube. The pressure drop from the inlet of the tube to the outlet of the tube is a direct measure of the viscosity of the fuel passing through the tube. A drawback of this arrangement is that line pressure surges, for example, caused as each engine injector is fired, vary drastically the pressure measurements across the capillary tube. The outlet end of the tube is exposed to such surges and, therefore, "sees" these surges, while the inlet end of the tube is isolated from these surges by the pump and the capillary tube.
U.S. Pat. No. 3,938,369 shows a viscometer similar to the one shown in U.S. Pat. No. 2,771,770, except that a second capillary tube is added in series with the first one in the later issued patent. The second tube serves to dampen pulsations at the outlet of the viscometer, whereby the first capillary tube, which is the measuring tube, is isolated from such pulsations at its inlet and outlet by the pump and the dampening capillary, respectively. Although the arrangement in U.S. Pat. No. 3,938,369 reduces the effect of line surges on the viscosity measurements, it fails to provide a complete solution to the problem. To the extent that the flow restriction through the pump is different from the flow restriction of the damping capillary tube, either when the viscometer is put into service or with wear on the pump, the inlet and outlet of the measuring capillary tube will be isolated from pulsations to different extents causing incorrect viscosity measurements.